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employees coming to the airport or air base are the most typical indirect emissions. An example
of motor vehicle emissions that are indirect emissions of an action is a planned airport capacity
increase that is expected to generate additional passenger traffic to the airport (i.e., caused by the
action and foreseeable). In this example, the indirect emissions would be the sum of the motor
vehicle emissions resulting from new passenger and employee traffic for an average commute
trip distance (as well as any emissions decrease due to a reduction in vehicle traffic). Vehicle
emissions while on the airport’s property would be included in direct emissions and would be
excluded from indirect emissions to avoid double counting.
This section summarizes the various types of emissions sources and models available to aid in
the calculation of an emission inventory.
5 Construction activity is short-term and temporary in nature. Examples include construction of a new or
modified runway, roadway, parking facility, or terminal. Emission sources, calculation methodologies,
and data inputs vary slightly when evaluating construction impacts.
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3.2 Emission Sources
3.2.1 Aircraft
The sources of aircraft emissions addressed in this handbook are: commercial aircraft, general
aviation and air taxi aircraft, and military aircraft.
Commercial aircraft are operated on a scheduled basis by civilian international, national, regional
and commuter air carriers. Commercial aircraft also are operated on an unscheduled basis by
civilian charter operators. In addition to aircraft, emission sources related to air carrier aircraft
operations include auxiliary power units (APUs) and ground support equipment (GSE), discussed
separately below.
General aviation aircraft are privately owned and operated on a non-scheduled basis at a variety
of facilities ranging from commercial airports to small privately owned airstrips. Air taxis are
non-air carrier commercial operators that fly scheduled service carrying passengers and freight
on a limited basis. The smaller general aviation and air taxi aircraft generally do not require
APUs and GSE.
Military aircraft are operated by the Department of Defense (DOD). Military aviation includes
the full spectrum or aircraft types, ranging from high performance jet fighters to large transports
to small piston engine aircraft. Most military aircraft operations occur at DOD-operated air
bases, but certain operations can take place at civilian airports as well. Examples of such activity
include National Guard or active duty military aircraft based at a civilian facility and military
transports shuttling personnel to a civilian airport. Emissions from military aircraft encompass
those occurring at both military and civilian facilities. Civil aircraft also may shuttle personnel
between a military and civilian facility. The emissions of civil aircraft operations at DODoperated
air bases also are attributed to the air base. Emission sources related to military aircraft
include APUs and GSE, discussed separately below.
A significant consideration for airport and air base emission inventories is that aircraft not only
operate on the ground but emit pollutants, during their flight in the atmosphere. Due to
atmospheric mixing, some of these emissions affect ground level pollutant concentrations. The
portion of the atmosphere that is completely mixed begins at the earth’s surface and may extend
to a height of a few thousand feet. The volume is often referred to as the mixing zone or
inversion layer. The height to which the mixing zone extends is called the mixing height. All
pollutant emissions in the mixing zone must be accounted for in a complete airport or air base
emissions inventory.
The aircraft operations of interest within the mixing zone are defined as those in the landing and
takeoff (LTO) cycle. The standard LTO cycle begins when the aircraft enters the mixing zone as
it approaches the airport on its descent from cruising altitude, lands and taxis to the gate. The
cycle continues as the aircraft taxis back out to the runway, takes off, and climbs out of the
mixing zone and back up to cruising altitude. The five specific operating modes in a standard
LTO are: approach, taxi/idle-in, taxi/idle-out, takeoff, and climbout. Most aircraft go through this
sequence during a complete standard operating cycle. Some aircraft and operations may go
through a slightly different sequence during a non-standard operating cycle. Non-standard
sequences combine or eliminate certain modes. For example, helicopters combine takeoff and
climbout modes.
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For a detailed discussion of the aircraft emissions calculation methodology and data inputs see
　
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